1. Field of the Invention
The present invention relates to flow measurement and control systems and proportional valves for providing variable programmed flow rates in fluids in such systems.
2. Description of the Related Art
So far as is known, existing liquid flow control valve devices for proportional flow control applications have certain shortcomings or inadequacies. Proportional solenoid operated valves or a diaphragm valves have rapid operating speed. However, there are problems in programming solenoid and diaphragm valves inexpensively for accurate flow control under widely varying flow conditions.
Rotatable control valves typically have good flow control resolution and can hold flow set points. However, rotatable control valves are slow to respond. Further, rotatable control valves have problems with achieving high resolution over a wide range of flow rates in a relatively small valve size. In addition, programming for accurate flow control over a wide variety of flow conditions is often a problem.
Valve designs other than rotatable control valves have included diaphragm actuation for flow control and generally used more expensive diaphragm structure. These other types of valves do not have, so far as known, programmable actuating features. These other types of valve designs did not have programmable flow characteristics over widely varying flow conditions; higher resolution; and inexpensive programming characteristics.
A particular problem has been present when the fluid was in the form of a slurry, such as a polishing slurry of the type used in the semiconductor industry. These types of slurries often contained abrasive materials. Over a service life, it was necessary to adjust the valve flow control settings to compensate for component wear because of the effect of the abrasive materials in the slurries.